SETI@home is not very GPU intensive. Their GPU application also utilizes CPU for some calculations, so a lot of data is shuffled between the CPU and the GPU which makes a 100% GPU workload unlikely.
There are BOINC applications which could easily outmatch the power consumption of LTC hashing, for example Primegrid GFN-22 (looking for very large prime numbers). But if shorter GFN tasks are failing on your GPUs, GFN-22 will more than likely fail too. It could be that those tasks are failing precisely because they are hard on the GPU.
Thanks, @vortac!
It looks that PrimeGrid works perfectly with the old GeForce GT630 in Ubuntu 16.04. I will let this one working on PrimeGrid for a while, and use the time to find a solution also for the HD7990.
Many thanks for your support!